The present invention relates to a tunnel effect magnetoresistance, also known as a xe2x80x9cmagnetic valvexe2x80x9d magnetoresistance, and a magnetic sensor using such a magnetoresistance.
Magnetic sensors are sensitive to magnetic fields or fluxes. In this way, the magnetic sensor according to the invention may be used, for example, to read data recorded on magnetic data storage media. In addition, the invention may be used to produce Magnetic Random Access Memory devices.
The magnetic sensor may also be used to determine an electric current flowing in a wire, by measuring the magnetic field applied in the vicinity of said wire.
Finally, other applications of the magnetic sensor, such as a position sensor or a magnetoresistive compass, may also be envisaged.
More generally, the invention relates to any type of sensor or magnetoresistance capable of detecting or measuring magnetic fields, particularly weak fields, i.e. ranging from a few A/m to a few thousand A/m.
Until recently, the magnetoresistive sensors used to detect weak magnetic fields, particularly in the field of magnetic recording, were mostly sensors based on a xe2x80x9cmagnetoresistance anisotropismxe2x80x9d effect.
The magnetoresistance anisotropism effect can be seen in ferromagnetic transition metals such as nickel, cobalt or iron-based alloys. It consists of a variation of the resistivity of the magnetic material as a function of an angle between an electric measurement current flowing through the material and the magnetisation of the material.
The relative change in resistivity xcfx81 of the magnetic material, referred to as xcex94xcfx81/xcfx81, may reach 4 to 5% at room temperature for fields of the order of 1 kA/m, and in solid ferromagnetic transition metals. However, this amplitude is reduced to 1 to 2% when the same materials are deposited in thin layers with thicknesses of 15 to 30 nanometer. This range of thicknesses is that used to manufacture current magnetoresistive sensors. Therefore, the sensitivity of these sensors is limited. In addition, their response is not linear. Indeed, the variation in the resistivity is proportional to the square of the cosine of the angle between the measurement current and the magnetisation.
Sensors operating according to a xe2x80x9cgiant magnetoresistancexe2x80x9d effect are also known. This effect was first discovered for iron-chromium type multilayer structures and subsequently for other multilayer systems formed by alternating layers of ferromagnetic transition metal and layers of non-magnetic metal.
In these systems, the magnetoresistance effect is essentially linked to a change in the relative orientation of the magnetisations of the successive ferromagnetic layers. This effect is usually referred to using the terms xe2x80x9cgiant magnetoresistancexe2x80x9d or xe2x80x9cspin-valve effectxe2x80x9d.
In spin-valve type magneto resistances, the sensitive ferromagnetic layer, i.e. the free magnetisation layer, has a thickness between 6 and 12 nm to obtain a maximum magnetoresistance amplitude. Below 6 nm, said magnetoresistances have a response amplitude that decreases considerably. Therefore, this type of magnetoresistance is limited in terms of sensitivity for low flux quantities.
Document 1, the reference of which is given at the end of the present disclosure, gives a very general description of the use of said giant magnetoresistance effect to produce magnetic field sensors.
Finally, it is known that there is a magnetoresistance effect in metal-insulating material-metal tunnel effect junctions wherein a thin layer of insulating material, forming a potential barrier for conduction electrons, is inserted between two layers of magnetic metal.
The magnetic metal is selected, for example, from Fe, Co, Ni or their alloys and the layer of the insulating material, a few nanometer thick, is composed of a material selected, for example, from Al2O3, MgO, AlN, Ta2O5, HfO2, NiO.
In this type of junction, when electrons are forced to pass through the barrier by means of a tunnel effect, by connecting the junction to a current source or by applying a voltage between the two layers of magnetic metal, it is observed that the conductance G of the junction varies as a function of the relative orientation of the magnetisations of the layers of magnetic material at either side of the barrier formed by the insulating material (in the manner of an optical polariser-analyser system).
This effect, called the xe2x80x9cmagnetic valve effectxe2x80x9d, was first observed only at low temperatures and its amplitude was very low.
However, specific magnetic material/insulating material/magnetic material type structures with Fe/Al/Al2O3/FeCo type junctions have made it possible to obtain variations in conductance, at room temperature, with an amplitude of the order of 17%.
Magnetic valve effect structures are described, for example, in documents 2, 3 and 4. Similarly, experiments on tunnel effect junctions are described in documents 5 and 6. The references of these documents are given at the end of the present disclosure.
Recently, considerable progress was made in the development of junctions, particularly in relation to the quality control of the insulating barrier.
The insulating barrier is produced, for example, by depositing a thin layer of aluminium on one of the metal electrodes of the junction and then oxidising the aluminium layer with oxygen plasma.
The oxygen plasma oxidation time thus makes it possible to check the thickness and, therefore, the electrical resistance of the insulating barrier.
It is also possible to allow the layer of aluminium to oxidise in air. In this case, the results and the quality of the insulating barrier are less reproducible.
In magnetic valve effect junctions with a magnetic material-oxide-magnetic material type structure, designated Mxe2x80x94Oxe2x80x94Mxe2x80x2, the magnetic materials are selected such that the magnetisation of one of the magnetic layers (e.g. Mxe2x80x2a) remains fixed in a given direction, in the range of fields to be measured, while the magnetisation of the other layer (M in this example) is capable of following the variations of the field applied. The first layer is called the xe2x80x9ctrapped layerxe2x80x9d while the second is called the xe2x80x9csensitive layerxe2x80x9d. The benefit of magnetic valve junctions in relation to spin-valve structures is that they offer wider measurement amplitudes (17% instead of 5 to 9%).
The invention relates to a tunnel effect magnetoresistance as described above offering a wider conductance variation amplitude.
The invention also relates to magnetoresistances with an increased sensitivity and offering a more compact size.
The invention also relates to a magnetic sensor, particularly for ultra-high density magnetic recording (greater than 10 Gbit/inch2), making it possible to read data using very small quantities of magnetic flux.
To achieve these objectives, the present invention more specifically relates to a tunnel effect magnetoresistance comprising, in the form of a stack:
a first layer of free magnetisation magnetic material,
a xe2x80x9cbarrierxe2x80x9d layer, composed of an electrically insulating material, and
a second layer of trapped magnetisation magnetic material.
According to the invention, the thickness of the first layer of magnetic material is less than or equal to 7 nm.
A particularly good magnetoresistance sensitivity may be obtained when the thickness of the first layer of magnetic material is between 0.2 nm and 2 nm.
Thanks to the extreme thinness of the first layer of magnetic material in particular, the magnetoresistance shows wide-amplitude conductance variations for low variation values of the magnetic flux applied.
Such a magnetoresistance is thus suitable for reading data on data media, such as hard disks, with a high data density. Indeed, the greater the density of data stored on a hard disk, the lower the quantity of magnetic flux xcfx86 produced by the magnetic transitions between two adjacent data bits, picked up by a read head, is. This magnetic flux induces a rotation xcex94xcex8 of the magnetisation of the sensitive magnetic layer given by xcfx86≈LLxc2x7EEmxc2x7xcex94xcex8. In this expression, Le represents the cross-section of the sensitive magnetic layer wherein the magnetic flux xcfx86 enters, e is the thickness of said sensitive layer and Ms is its spontaneous magnetisation. The above expression implies that, at an equal quantity of flux xcfx86, the lower the thickness e of the sensitive layer, the greater the rotation xcex94xcex8 of its magnetisation is.
The conductance G(xcex94xcex8) of the junction varies according to the formula:       G    ⁡          (      Δθ      )        =            G      antiparallel        +                  (                              G            parallel                    -                      G            antiparallel                          )            ⁢              (                              1            +                          cos              ⁡                              (                Δθ                )                                              2                )            
In this formula, Parallel and Antiparallel represent the conductances in the parallel and anti-parallel configurations, respectively. It appears that the higher xcex94xcex8 is, the greater the conductance variation is and, therefore, the greater the sensitivity of the sensor is.
To compare with known conventional structures, it should be pointed out that, in metal magnetoresistive sensors based on the magnetoresistance anisotropism effect, or AMR, the thickness of the sensitive layer is at least 15 nm. In such sensors, it is not possible to reduce the thickness of the sensitive layer significantly. Indeed, in AMR sensors, the amplitude of the conductance variation effect falls significantly when the thickness of the sensitive layer is less than 20 nm. In the same way, the electrodes used in tunnel effect junctions, as described in documents 5 and 6, for example, have considerable thicknesses, greater than 30 nm.
According to a particular aspect of the invention, the second magnetic layer, with trapped magnetisation, may be produced with a thickness comparable to that of the first magnetic layer. For example, its thickness is between 0.4 and 2 nm.
When the second magnetic layer is thin, the magnetostatic interactions between the second layer, in which the magnetisation is trapped, and the first layer, in which the magnetisation is free, are low. In this way, the first magnetic layer is not influenced and retains its free magnetisation property better, even in very small magnetoresistance structures.
According to another aspect of the invention, the magnetoresistance may also comprise a layer of non-ferromagnetic metal NM between the first layer of magnetic material M and the insulating barrier O. The purpose of this layer NM is to form an anti-reflection coating for only one category of conduction electrons (or the spin electrons parallel to the magnetisation of the layer M, or the spin electrons anti-parallel to the magnetisation of M). This results in a high increase in the effective polarisation of the electrons passing through the insulating barrier by means of a tunnel effect and thus a greater magnetoresistance amplitude. This effect is described from a theoretical point of view in document 7. The physical origin of this spin-dependent anti-reflection effect is the same as that giving rise to magnetic coupling oscillations through the non-ferromagnetic layers observed in multilayer structures of a period M/NM when the thickness of the layers NM varies. It lies in the fact that the electron reflection coefficients at the N/NM interfaces depend on the electron spin in relation to the magnetisation of the layer M. For more information on this subject, it is possible to refer to documents 8 and 9 listed at the end of the present disclosure. Consequently, to benefit from this selective spin anti-reflection effect in the tunnel junctions according to the invention, it is necessary to select the magnetic metal/non-ferromagnetic metal pair such that the corresponding multilayers of the period M/NM show coupling oscillations as a function of the thickness of the layers NM. Considerable literature is currently available on these coupling oscillation multilayers (e.g. see document 8). This literature may be used as a database for selecting the materials M/NM. A particularly appropriate choice for tunnel junctions consists of using a magnetic metal layer made of Co or Co-rich Co1xe2x88x92xFex alloys (x between 0 and 50%) and a layer of Cu for the non-ferromagnetic layer. Indeed, Co/Cu multilayers are known to show significant coupling oscillations through Cu. Other examples of possible choices of N, NM pairs are (Fe, Cr), (Co, Ru) and (Fe, Au) . The thickness of the layer NM may vary from 0 to approximately 10 nm and preferably from 0.04 to 3 nm.
These anti-reflection layers NM may advantageously be inserted at either side of the insulating barrier O between the magnetic layers and the insulating barrier.
The magnetoresistance may also comprise at least one layer of magnetic material, or doping layer, inserted at the interface between the magnetic layer M and the layer NM or, failing a layer NM, between M and the insulating barrier O.
The doping layers are very thin layers making it possible to increase the polarisation of the electrons passing through the barrier layer by means of the tunnel effect.
They may be composed of cobalt, iron or a Co1xe2x88x92xFex alloy, where x is a parameter between 0 and 1. If required, the doping layers may contain low quantities of other elements, such as nickel.
The invention also relates to a magnetic sensor, for example for reading data, comprising one or more magnetoresistances as described above.
The invention""s other characteristics and advantages will be seen more clearly in the following description, with reference to the appended figures. This description is given solely for illustration purposes and is not exhaustive.